Method for storage and release of hydrogen

ABSTRACT

The invention provides a process for the production of hydrogen, comprising catalytically decomposing a concentrated aqueous solution of potassium formate in a reaction vessel to form bicarbonate slurry and hydrogen, discharging the hydrogen from said reaction vessel, and treating a mixture comprising the bicarbonate slurry and the catalyst with an oxidizer, thereby regenerating the catalyst. Pd/C catalysts useful in the process are also described.

FIELD OF THE INVENTION

The invention relates to a method for providing hydrogen in a storableand transportable form, based on the bicarbonate-formate cyclic system.

BACKGROUND OF THE INVENTION

The bicarbonate-formate cycle has been described by Zaidman, Weiner andSasson [Int. J. Hydrogen Energy, 11(5), pp. 341-347 (1986) and Weiner,Blum, Feilchenfeld, Sasson and Zalmanov [Journal of Catalysis, 110, pp.184-190 (1988)], suggesting the use of aqueous formate solutions ashydrogen carriers. The bicarbonate-formate cycle consists of two stages,as shown by the following chemical equation:HCO₃ ⁻+H₂↔HCO₂ ⁻+H₂O

The first stage involves the reduction of bicarbonate to formate. Tothis end, a bicarbonate salt in aqueous solution is reacted withhydrogen at about 35° C. under hydrogen pressure, to give an aqueoussolution of the corresponding formate salt. On demand, the reversereaction is carried out, normally at about 70° and atmospheric pressure,whereby the formate is decomposed to produce the bicarbonate andhydrogen. The hydrogen can then be used for any desired purpose, e.g.,as a fuel material. It follows that in the first stage (formatesynthesis), the system is loaded with hydrogen, which is released anddelivered in the second stage (formate decomposition). Both stages arecarried out in the presence of a catalyst, e.g. heterogeneous catalystsuch as palladium.

Kramer, Levy and Warshawsky [Int. J. Hydrogen Energy, 20(3), pp. 229-233(1995)] investigated the activity of the catalysts used inbicarbonate-formate cycle, starting by reacting 3.5 M KHCO₃ solutionwith hydrogen to give the formate. The authors reported that theactivity of the palladium catalyst used decreases with time,demonstrating that the catalyst can be regenerated through the followingsequence of steps: (i) separating the catalyst from the solution; (ii)washing with distilled water at ambient temperature; (iii) drying at120° C. under argon atmosphere; (iv) oxidizing the catalyst with oxygenor air.

It would be beneficial to provide a process allowing enhanced hydrogenstorage and production capacity, and on the same time, offering aconvenient way for treating and regenerating the catalyst used.

THE INVENTION

The dependence of solubility on temperature was investigated forpotassium bicarbonate and potassium formate in the temperature rangesfrom 0 to 70° C. and 0 to 90° C., respectively. The solubility curvesare graphically presented in FIG. 1, indicating that potassiumbicarbonate is significantly less soluble in water in comparison topotassium formate. The results suggest that the reduced solubility ofpotassium bicarbonate in comparison to potassium formate may introduce aserious limitation into the bicarbonate-formate system from theperspective of hydrogen storage capacity.

The inventors found that the catalytically-driven bicarbonate-formatestorage cycle can be generated starting with a bicarbonate slurry (inlieu of a bicarbonate solution), which upon reaction with hydrogen giveshighly concentrated aqueous formate solution. The catalyticdecomposition of this formate solution yields back the bicarbonateslurry and hydrogen. Surprisingly, the bicarbonate slurry, whichconsists of a solid mixture of the bicarbonate salt, the catalyst and asmall amount of water, permits an easy regeneration of the catalyst. Theregeneration is accomplished by exposing the slurry to air or oxygen,e.g., at elevated temperature under vigorous mixing, whereby thecatalyst regains its activity.

By the term “bicarbonate slurry”, as used herein, is meant one or morebicarbonate salts in a solid form (e.g., solid KHCO₃), preferably in amixture with water. The weight ratio between the solid bicarbonatecomponent and the aqueous phase of the bicarbonate slurry is preferablynot less than 1:1 and more preferably not less than 2:1.

The present invention is therefore directed to a process for theproduction of hydrogen, comprising catalytically decomposing aconcentrated aqueous solution of potassium formate in a reaction vesselto form bicarbonate slurry and hydrogen, discharging the hydrogen fromsaid reaction vessel, and treating a mixture comprising the bicarbonateslurry and the catalyst with an oxidizer, thereby regenerating saidcatalyst. Formate decomposition to generate hydrogen can be advancedunder heating, but also at ambient temperature, in an acidicenvironment. More specifically, the decomposition of formate takes placeat a temperature above 50° C., e.g., in the range 50 to 70° C., or at atemperature below 50° C., e.g., in the range 0 to 45° C., with the aidof an acid.

The present invention is also directed to a process for the storage ofhydrogen, comprising treating a mixture of potassium bicarbonate slurryand a catalyst with an oxidizer, and catalytically reducing saidbicarbonate slurry in a reaction vessel to form a concentrated aqueoussolution of potassium formate.

Another aspect of the present invention relates to the use ofbicarbonate slurry in admixture with catalyst particles in theheterogeneous catalytic hydrogenation of bicarbonate to formate. Theslurry is periodically treated with an oxidizer, whereby the catalyst isregenerated.

More specifically, the present invention is directed to a process forstorage and subsequent release of hydrogen, comprising:

(i) catalytically hydrogenating in a reaction vessel a potassiumbicarbonate slurry to form a concentrated aqueous solution of potassiumformate;

(ii) catalytically decomposing said formate solution in a reactionvessel to form a bicarbonate slurry and hydrogen;

(iii) discharging the hydrogen from said second reaction vessel, and

(iv) treating a mixture comprising said bicarbonate slurry and thecatalyst with an oxidizer, thereby regenerating said catalyst.

The process of the invention involves the synthesis of an aqueoussolution of potassium formate through the heterogeneous catalyticreduction of a potassium bicarbonate slurry. To this end, thebicarbonate salt and water are charged into a suitable reaction vessel,followed by the addition of a catalyst. It should be noted, however,that the catalyst normally contains some water. The water content of thecatalyst may suffice for the purpose of slurry formation, such that theaddition of water to the reaction vessel is unnecessary. The reactionvessel is capable of withstanding high pressures (e.g., high pressureautoclave). An especially preferred bicarbonate salt is potassiumbicarbonate. The amounts of the bicarbonate salt and water are adjustedas set forth above, forming a slurry in the reaction vessel.

Catalysts operable in the process include palladium or supportedpalladium, e.g., palladium supported on carbon. Palladium on PANI(polyaniline), palladium on CNT (carbon nano tubes) and palladium onmontmorillonite treated in ionic liquid (montmorillonite suspended inionic liquid and then filtered before being used as palladium support)are also useful in the process of the invention. The preparation ofdifferent forms of supported palladium catalysts is illustrated in theworking examples below. Suitable catalysts are also commerciallyavailable from Engelhard, Johnson Matthey, and Sigma-Aldrich. The molarratio between the bicarbonate salt and the catalyst is in the range from50:1 to 1000:1, preferably about 200:1 to 700:1.

The conversion of the bicarbonate to formate is accomplished in thepresence of hydrogen. Thus, hydrogen is fed to the reaction vessel at atemperature in the range from 25° C. to 70° C., preferably at about 35°C., to pressure of about 4-25 atmospheres. The reaction mixture ismaintained under stirring for not less than 1 hour, e.g., about 2 hours,thereby completing the formate synthesis. The concentration of theformate salt in the resulting solution is not less than 4M, preferablynot less than 5M and more preferably not less than 8M, and may be up tosaturation.

For example, from 8.0 to 15.7 M (measured at room temperature; thesolubility limit of formate at 70° C. is 16 M). When terms such as “aconcentrated aqueous solution of potassium formate” are used herein,then a solution with the concentration characteristics set forth aboveis intended (e.g., with concentration of not less than 4M, preferablynot less than 5M, and more preferably not less than 8M, etc.).

It should be noted that the hydrogen employed in the first stage of theprocess may be either from a commercially available cylinder, in whichcase the process of the invention is chiefly utilized for converting thegaseous hydrogen into a “latent” form, i.e., the formate aqueouscarrier, which is more easy and safe to handle for storage andtransportation. However, the hydrogen may be produced in situ, e.g., bymeans of electrochemical methods, following which the hydrogen whichevolves on the electrode is directly absorbed by the bicarbonate slurry.

The process of the invention involves the decomposition of the aqueousformate solution, for generating hydrogen. It should be understood thatin many cases, the synthesis and decomposition of the formate are bothcarried out in the same reaction vessel (namely, in a single reactionvessel). However, in those cases where the formate solution is producedin one place and subsequently transferred to another place, i.e., to asite of its intended use, then the reaction vessels used for storage andrelease of hydrogen may be different.

The decomposition of the formate is carried out under atmosphericpressure at a temperature which is preferably not less than 0° C., e.g.,in the range 0 to 45° C., with the aid of an acid (for example, at atemperature from 15° C. to 30° C.), or in the range from 50° C. and 70°C. when the process is devoid of acidification. The decompositionreaction lasts not less than 30 minutes, e.g., about 45 minutes. Themolar ratio between the formate salt and the catalyst is in the rangefrom 50:1 to 1000:1, preferably about 200:1 to 700:1.

Upon completion of the decomposition step, the hydrogen gas produced isdischarged from the reaction vessel and is delivered to, and utilizedin, for example, an electricity-generating system, e.g., a fuel cellinvolving the use of hydrogen. An example is illustrated in FIG. 3,which is described below.

Following hydrogen discharge, the reaction mass left in the reactorconsists of a bicarbonate slurry in admixture with the catalystparticles. The slurry is treated with an oxidizer, which is mostpreferably air or oxygen. The oxidizer gas is fed to the reactor topressures of not less than 1 atm., e.g., from 1 atm. to 10 atm., and thecontent of the reactor is thoroughly mixed, such that the catalystparticles are exposed to the oxidizer. For example, the mixing can becarried out using a screw impeller. The regeneration step is preferablycarried out at a temperature of not less than 50° C., for at least 60minutes. Following the regeneration step, the slurry may be used in thesynthesis of formate aqueous solution, as set forth above. It should benoted that the foregoing catalyst treatment is carried out periodically,i.e., the reversible catalytic hydrogenation and dehydrogenation areallowed to run in a cyclic manner for several times until the need forcatalyst regeneration arises.

FIG. 3 schematically illustrates one embodiment of the process of theinvention. In the first stage (formate synthesis), hydrogen generated inan electrolytic cell (1) is compressed in a compressor (2) to thedesired working pressure and then fed via feed line (9) into a reactionvessel (3) charged with a bicarbonate slurry and a catalyst. Thehydrogen is fed to pressure of up to 25 atmospheres. The reaction vesselis equipped with suitable agitation means and heating means. The formatesynthesis is carried out under the conditions set forth above. Ondemand, the formate is decomposed under atmospheric pressure to formhydrogen and bicarbonate slurry in admixture with the catalystparticles. The hydrogen is released from the reactor through pipe (4)and delivered to a fuel cell (5), where it is oxidized (e.g., throughthe use of oxygen) to generate an electric current. Following hydrogenremoval from the reactor and prior to the next step of formatesynthesis, air is introduced into the reactor to pressure of up to 10atmospheres, from air compressor (6), via feed line (10). Under throughmixing, the catalyst is oxidized and regenerated such that it caneffectively catalyze a subsequent formate formation reaction.

The process described above involves the conversion of a bicarbonateslurry into highly concentrated aqueous solution of the correspondingformate salt, from which hydrogen can be subsequently liberated (e.g.,potassium bicarbonate→potassium formate). However, it has been alsofound that hydrogen can be effectively stored and released from aformate slurry, e.g., from a sodium formate slurry (sodium formate ismuch less soluble in water than potassium formate). Thus, the inventionalso relates to a process comprising catalytically decomposing a formateslurry, to form the corresponding bicarbonate and hydrogen. Followingthe release of hydrogen, the resultant mixture of bicarbonate andcatalyst particles can be treated with air as described above, in orderto refresh the catalyst.

We have also investigated the effect of acid addition on thedehydrogenation of potassium formate over Pd/C. It should be noted thatacidification of the reaction medium causes the following undesiredreaction of bicarbonate decarboxylation (equation no. 2):1. HCO₂ ⁻+H₂O

HCO₃ ⁻+H₂2. HCO₃+H₃O⁺

2H₂O+CO₂

The result of bicarbonate decarboxylation is a loss of CO₂ andirreversibility for hydrogen storage. In order to store hydrogen againwe have to supply CO₂ and invest energy to recover bicarbonate fromcarbonate.

We have found that the process is still manageable in an acidicenvironment to achieve useful results which compensate for some decreasein the storage capacity of the system caused by the evolvement and lossof carbon dioxide. On selection of suitable acidic conditions, thedehydrogenation of potassium formate over Pd/C occurs at surprisinglyincreased rates at reduced temperature (T<70° C., e.g., from 0 to 50°C.). The enhanced reaction rates attainable in an acidic environmenteven at ambient temperature lead to a rapid generation of hydrogen ondemand. Thus, instantaneous hydrogen release is possible on account ofthe fact that there is no need to apply heating to accelerate thereaction. The hydrogen gas which rapidly evolves on acidification of theformate solution can be utilized at once as a fuel material in a fuelcell coupled to systems in need for an immediate power supply, e.g.,emergency backup electrical generators powered by fuel cells.

The properties considered important for screening potentially usefulacids for acidifying the formate solution are the turnover numbers atdeactivation (TONs) and turnover frequencies (TOFs) measured for thecatalytically-driven formate decomposition reaction, indicative of thecatalyst activity. In the experimental work reported below, a variety ofacids were tested for their ability to advance the reaction, namely,mineral acids such hydrogen chloride, nitric acid and sulfuric acid, andorganic acids such as acetic acid and formic acid, with the latteremerging as the best choice.

The acidification of the reaction medium can be achieved by addition ofan acid to the aqueous formate solution, by incorporation of a solidacid into the heterogeneous catalyst (can be used in a continuousreactor) or by acid treatment to the carbon support of the palladiumcatalyst. In general, the pH of the aqueous formate solution is adjustedwithin the range from 3 to 6.

In the presence of an acid, especially formic acid, the decomposition ofpotassium formate proceeds efficiently to generate hydrogen at atemperature below 50° C., e.g., in the range from 0 to 45° C. Forexample, within the temperature range below room temperature, i.e., lessthan 15° C., TON and TOF exceeding 500 and 20, respectively, weremeasured, as illustrated by the experimental work reported below. Itshould be noted that even highly concentrated potassium formatesolutions can benefit from the presence of an acid, i.e., theacidification allows a swift production of hydrogen within thetemperature range of 15 to 45° C. starting with solutions havingpotassium formate concentration as high as 8M-15M, e.g., 10M-13M.

The molar ratio between potassium formate and formic acid appears to bean important process variable. On plotting the TON measured (atdeactivation) against the molar ratio potassium formate:formic acid(abbreviated MR_(PF:FA)), a curve resembling the graphic description ofan inverted parabola is obtained, with an axis of symmetry lying in therange from 10:1≤MR_(PF:FA)≤10:10. The exact MR_(PF:FA) value for which amaximal TON is measured may depend on factors such the temperature ofthe reaction and concentration of the potassium formate solution. Forexample, for potassium formate decomposition taking place at about roomtemperature, high TON (e.g., exceeding 500) are achieved when theMR_(PF:FA) is adjusted within the range from 10:2 to 10:6 for 5M to 15Mpotassium formate solution.

Accordingly, another aspect of the invention is a process for theproduction of hydrogen, comprising catalytically decomposing potassiumformate in a concentrated aqueous solution in the presence of an acid,which is preferably formic acid, at a temperature below 50° C., e.g.,from 0 to 45° C. (for example, from 15 to 30° C., i.e., at ambienttemperature), to form bicarbonate slurry and hydrogen, discharging thehydrogen from said reaction vessel, and treating a mixture comprisingthe bicarbonate slurry and the catalyst with an oxidizer, therebyregenerating said catalyst. The concentration of the aqueous potassiumformate solution is not less than 4 M, and the molar ratio potassiumformate to the acid is from 10:1 to 10:10, preferably from 10:2 to 10:6.

In one embodiment of the invention, the process comprises the step ofadding an acid (e.g., HCOOH) to the concentrated potassium formatesolution and carrying out the decomposition reaction at ambienttemperature, i.e., without heating the potassium formate solution.Returning to the apparatus illustrated in FIG. 3, the addition of theacid may be actuated in response to a signal generated when there is ademand for an immediate hydrogen supply to a fuel cell (5). The acidheld at a tank (7) is then fed to the reaction vessel (3) through a feedline (8). For example, in response to the issuance of an alarm signal(not shown), a metered amount of the acid is injected into the reactionvessel, driving formate decomposition and hydrogen production at roomtemperature. The added acid may be supplied to the reaction mixture inthe form of an aqueous solution or in a solid form through a soliddosing pump.

As previously explained, using an excessive amount of an acid may leadto the loss of the reversibility of the bicarbonate-formate cycle due toCO₂ evolvement. However, the reaction according to equation (2) does notoccur, or is at least minimized, when the acid used is formic acid.

In another variant of the process, the decomposition is started at afirst temperature T₁ below 50° C. in the presence of an acid (e.g.,added HCOOH), and on consumption of the acid, the reaction vessel isheated to a second temperature T₂ above 50° C., whereby the reaction offormate decomposition reaches completion at a temperature above 50° C.

We have also tested the performance of several palladium on carbonsupport (Pd/C) catalysts in connection with the bicarbonate-formatecycle and found that high hydrogen storage and production capacity couldbe achieved with the aid of Pd/C catalyst with Pd loading in the rangefrom 0.15 to 1.0 wt %, preferably 0.2 to 0.5 wt %, characterized in thatat least a portion of the palladium is present on the support in theform of sub-nanometer particles (<1 nm). The presence of thesub-nanometer Pd particles in the catalyst sample is indicated byScanning Transmission Electron Microscopy with Energy Dispersive X-raySpectroscopy (STEM-EDS), identifying palladium-containing regions in thecarbon support, which regions consist of invisible Pd particles (i.e.,below TEM resolution and therefore too small to be visible in the TEMimage). Additionally, Pd particles in the low nanometer range of size(from 1 nm to 20 nm, e.g., from 1 nm to 5 nm) are also present in thePd/C catalyst; these particles are visible in TEM images.

Pd/C catalyst with the properties set forth above can be prepared byreduction of palladium salt (e.g., Pd(NO₃)₂·2H₂O) using a mild reducingagent (for example, potassium formate) over activated carbon. Morespecifically, the Pd/C catalyst is prepared by a process comprisingdissolving in water a palladium salt, such as palladium (II) nitratedihydrate, adding to the solution heat-treated activated carbon(preferably a form bearing acidic groups, such as C-3345 available fromSigma), stirring the so-formed mixture, reducing the Pd²⁺ to Pd⁰ undermild conditions (e.g., with the aid of formate, especially potassiumformate, as a reducing agent), collecting a powder consisting of Pd/C,washing and drying same.

The reduction step preferably takes place at room temperature. Potassiumformate is added to the reaction vessel preferably gradually, e.g. overa period of time of not less than 15 minutes, such that theconcentration of potassium formate in the reaction mixture is less than0.15M, with a concentration from 0.001 to 0.12M, e.g., from 0.005 to0.01M, being preferred. For example, the molar ratio between the formateand the palladium salt 1:1 to 20:1.

The so-formed Pd/C catalyst, with Pd loading in the range from 0.15 to1.0 wt %, preferably 0.2 to 0.5 wt %, displays higher activity thancommercially available Pd/C 5% catalyst (i.e., with higher Pd loading).The experimental results reported below indicate that hydrogen can bereleased from potassium formate solution over Pd/C 0.2% of the inventionwith better TON than when using Pd/C 5%.

The compositional information obtained by the imaging techniquesemployed for characterizing the Pd/C samples indicates that potassium(from the reductant) is incorporated into the Pd/C catalyst. Thepotassium appears to be found in the vicinity of Pd particles of the lownanometer range of size. Regions of the Pd/C where the sub-nanometer Pdis present appear to be potassium-free.

The concentrated aqueous formate solution (i.e., with concentrationhigher than 5M, e.g., from 8M and up to saturation) obtainable from abicarbonate slurry as described above is a liquid carrier capable ofstoring hydrogen and releasing same on demand at a site of use. Thehydrogen can be put to use within a power system or a vehicle, but alsoin other applications, such as for filling balloons and airships in thefield.

Another aspect of the invention is a power system comprising at leastone fuel cell and a hydrogen-generating unit for delivering hydrogen tothe anodic compartment of said fuel cell, said hydrogen-generating unitcomprising a catalyst-containing composition capable of reversiblecatalytic hydrogenation and dehydrogenation, which composition is in theform of potassium bicarbonate slurry and a concentrated aqueouspotassium formate solution, respectively, wherein saidhydrogen-generating unit is provided with a first feed line forintroducing an incoming hydrogen stream for hydrogenating said potassiumbicarbonate slurry and a second feed line for introducing an oxidizer(e.g., a stream of pressurized air) into said reaction vessel forregenerating said catalyst, and a discharge line for directing ahydrogen stream generated on dehydrogenating said potassium formatesolution to said fuel cell.

FIG. 3 is a schematic illustration of a specific embodiment of thehydrogen-generating unit which could be coupled to a fuel cell (5). Asingle reaction vessel is used for holding the catalyst-containingcomposition capable of reversible catalytic hydrogenation anddehydrogenation. The reaction vessel is a pressure reactor (3) equippedwith heating and agitation means. The hydrogen-generating unit furthercomprises a hydrogen source (1) and means for pressurizing hydrogen (2)connected through feed line (9) to the reaction vessel (3). Feed line(10) is used to deliver an oxygen-containing gas, preferably pressurizedair, to said pressure reactor (3). As shown in FIG. 3, feed lines (9)and (10) may be joined into a single line. The hydrogen-generating unitmay further comprise an acid storage tank (7), for injecting an acidthrough feed line (8) into the reactor (3). Hydrogen stream releasedfrom the composition is guided through line (4) to the fuel cell (5).

IN THE DRAWINGS

FIG. 1 shows the solubility curves of potassium bicarbonate andpotassium formate.

FIG. 2 is a graph showing the effect of catalyst regeneration on thehydrogenation-dehydrogenation cycle.

FIG. 3 schematically illustrates an apparatus for carrying out theprocess of the invention.

FIGS. 4A and 4B are graphs showing the effect of catalyst regenerationon the hydrogenation-dehydrogenation cycle.

FIG. 5 is a bar diagram showing TON and TOF measured in 4M potassiumformate solution with different amounts of formic acid, at 70° C.

FIG. 6 is a bar diagram showing TON measured in 4M potassium formatesolution with different amounts of formic acid, at room temperature.

FIG. 7 is a plot of TON against time on dehydrogenating differentconcentrated potassium formate solutions in the presence of formic acid,at room temperature.

FIG. 8 is a curve showing the TON (left ordinate) and percentage offormate conversion (right ordinate) as function of reaction time,measured at two different temperatures.

FIG. 9 is a bar diagram showing the dehydrogenation of potassium formatesolution in the presence of formic acid at different temperatures.

FIG. 10 is a graph showing TON measured on dehydrogenating potassiumformate solution with different Pd/C catalysts.

FIG. 11 is a STEM picture of Pd/C 0.2% prepared by formate reduction.

FIG. 12 is a STEM picture of Pd/C 0.2% prepared by formate reduction.

EXAMPLES Example 1 Reversible Hydrogen Absorption Over PotassiumBicarbonate Slurry

23.8 g of Pd/C 5% (51.3% wet, Engelhardt 5.47 mmol) were placed in anautoclave vessel along with 28.9 g of potassium bicarbonate (0.29 mol)and 12.23 g of water. The autoclave was sealed and washed with nitrogengas 3 times.

Hydrogen was added to the autoclave at 35° C. to 9.5 atmospheres andmixed for at least 2 hours. Then the initial pressure was released andgas flow from the autoclave was recorded while heating to 70° C. Thiscyclic procedure (formate synthesis and formate decomposition) wasrepeated 7 times without opening the autoclave. Then air was added tothe autoclave to 10 atmospheres and heated to 70° C. for 2 hours withstirring in order to refresh the catalyst. After air addition, theautoclave was washed with nitrogen followed by hydrogen. Again hydrogenwas loaded and its release was recorded for 5 more rounds, followed bycatalyst regeneration under the conditions set forth above. The cyclicprocedure was repeated again up to a total of 21 rounds, with thecatalyst regeneration step taking place after the seventh, twelfth andseventeenth rounds. The results are graphically presented in FIG. 2, forthe 2^(nd), 8^(th), 12^(th) and 18^(th) cycles, showing that followingthe catalyst regeneration according to the treatment of the invention(i.e., the 8^(th) and 18^(th) cycles), the rate of the reaction issignificantly increased.

Example 2 Reversible Hydrogen Absorption Over Potassium BicarbonateSlurry Using Pd/C 0.4% Catalyst

Potassium bicarbonate slurry↔10M potassium formate solution

A 300 ml autoclave was fed with 4 g of Pd/C 0.4% of Preparation 6 (60%wet, 0.06 mmol), 3 g of potassium bicarbonate (0.03 mol, Sigma 23705)and 1.125 g of deionized water (0.0625 mol). The molar ratio betweenpalladium and bicarbonate is 500:1. The molar ratio between water andbicarbonate is 2.1:1 and it fits the molar ratio between water andformate in a 10M potassium formate solution (the bicarbonate at theseconditions is in the state of slurry). The autoclave was purged 3 timeswith nitrogen gas before hydrogen was allowed to flow into it to apressure of 9.6 bar. The temperature was set to 35° C. and mechanicalstirring (cross impeller) was activated at 400 rpm for 2 hours. After 2hours the pressure was 8.3 bar. Then the autoclave's faucet was openedto reach rapidly to atmospheric pressure. The autoclave was connected toa water based flow-meter through a Ba(OH)₂ trap and heated to 70° C. torelease hydrogen.

The foregoing hydrogenation-dehydrogenation procedure was carried outtwo times, and then the catalyst was reactivated.

Catalyst reactivation: at the end of dehydrogenation the autoclave waspurged 3 times with nitrogen at 10 bar and then air was allowed to flowinto it to a pressure 10 bar. The autoclave was heated to 70° C. for 2hours and mechanical stirring (cross impeller) was set to 400 rpm. Thenthe autoclave's faucet was opened to reach rapidly to atmosphericpressure. The autoclave was purged 3 times with nitrogen at 10 barbefore it was charged with hydrogen to a pressure of 9.6 bar and heatedto 35° C. for 2 hours. Mechanical stirring (cross impeller) wasactivated at 400 rpm. Then the autoclave's faucet was opened to reachrapidly to atmospheric pressure. The autoclave was connected to a waterbased flow-meter through a Ba(OH)₂ trap and heated to 70° C. to releasehydrogen.

After catalyst reactivation, the cyclic hydrogenation-dehydrogenationprocedure was repeated four times, and then the step of catalystreactivation took place again. Thus, a total of seven cycles were run,with the catalyst regeneration step taking place after the second andsixth rounds. The results are presented in FIG. 4A. The upper linerepresents bicarbonate concentration and the lower line representsformate concentration.

Potassium bicarbonate slurry↔15.7M potassium formate solution

A 300 ml autoclave was fed with 26.5 g of Pd/C 0.4% of Preparation 6(60% wet, 0.4 mmol) and 5 g of potassium bicarbonate (0.05 mol, Sigma23705). The molar ratio between palladium and bicarbonate is 125:1.Hydrogenation of solid bicarbonate to formate without addition of watercan theoretically produce 15.7M potassium formate solution (in case allthe bicarbonate is hydrogenated to formate and water). The autoclave waspurged 3 times with nitrogen gas before hydrogen was allowed to flowinto it to a pressure of 9.6 bar. The temperature was set to 35° C. andmechanic stirring (cross impeller) was activated at 400 rpm for 2 hours.After 2 hours the pressure was 8.5 bar. Then the autoclave's faucet wasopened to reach rapidly to atmospheric pressure. The autoclave wasconnected to a water based flow-meter through a Ba(OH)₂ trap and heatedto 70° C. to release hydrogen.

The foregoing hydrogenation-dehydrogenation procedure was carried outfour times, and then the catalyst was reactivated.

Catalyst reactivation: at the end of dehydrogenation the autoclave waspurged 3 times with nitrogen at 10 bar and then air was allowed to flowinto it to a pressure 10 bar. The autoclave was heated to 70° C. for 2hours and mechanical stirring (cross impeller) was set to 400 rpm. Thenthe autoclave's faucet was opened to reach rapidly to atmosphericpressure. The autoclave was purged 3 times with nitrogen at 10 barbefore it was charged with hydrogen to a pressure of 9.6 bar and heatedto 35° C. for 2 hours. Mechanical stirring (cross impeller) wasactivated at 400 rpm. Then the autoclave's faucet was opened to reachrapidly to atmospheric pressure. The autoclave was connected to a waterbased flow-meter through a Ba(OH)₂ trap and heated to 70° C. to releasehydrogen.

After catalyst reactivation, the cyclic hydrogenation-dehydrogenationprocedure was repeated three times. Thus, a total of seven cycles wererun, with the catalyst regeneration step taking place after the fourthround. The results are presented in FIG. 4B. The upper line representsbicarbonate concentration and the lower line represents formateconcentration.

Examples 3 to 9 Formate Decomposition in the Presence of an Acid

To investigate the effect of acidic pH and type of acid on formatedecomposition, various acids were added to potassium formate (KHCO₂,abbreviated “PF”) 4M aqueous solutions in different acid: PF molarratios and the so-formed acidic solutions went through dehydrogenationover Pd/C at 70° C. (commercial Pd/C 5%, Sigma 205680). The conditionsof the reactions and the performance of the catalyst in the acidicenvironment are tabulated in Table 1.

TABLE 1 Ratio TON at TOF Example Acid (KHCO₂:acid:Pd) pH deactivation(min⁻¹) 3 HCOOH 2000:2000:1 4 264 62 4 HCOOH 2000:200:1 5 1131 93 5 HCl2000:1000:1 4 172 12 6 HNO₃ 2000:800:1 4 330 124 7 CH₃COOH 2000:5000:14.5 288 64 8 HNO₃ 2000:200:1 6 475 91 9 H₂SO₄ 2000:200:1 6 289 78

In the absence of an acid, the TON and TOF were ˜850 and ˜30,respectively. The experimental results set out in Table 1 indicate thatthe addition of an acid leads to increased TOFs, but in some cases theTONs were lower than can be achieved at neutral media. Formic acid(abbreviated “FA”) emerges as especially useful acid for enhancing thedecomposition of the formate to give hydrogen. In the experimental workto follow, formic acid was chosen for acidifying the formate solution.

Examples 10 to 16 Formate Decomposition in the Presence Varying Amountsof Formic Acid

The following set of experiments illustrates the effect of the molarratio between potassium formate and formic acid on formatedecomposition. In the tested solutions, the concentration of potassiumformate was 4M and the molar ratio potassium formate to palladiumcatalyst was constant (2000:1). Various amounts of FA were added tothese PF 4M aquatic solutions, which went through dehydrogenation overPd/C 5% (Sigma 205680) at 70° C. The conditions of the reactions and theperformance of the catalyst in the presence of formic acid are tabulatedin Table 2.

TABLE 2 Ratio TON at Example (KHCO₂:HCOOH:Pd) deactivation TOF (min⁻¹)10 (comparative) 2000:0:1 846 34 11 2000:20:1 914 47 12 2000:100:1 83883 13 2000:200:1 1131 86 14 2000:350:1 716 102 15 2000:500:1 405 102 162000:2000:1 264 62

The results are also presented in the form of a bar diagram in FIG. 5,where thick and narrow bars indicate the TONs and TOFs, respectively(the left and right ordinate correspond to the TON and TOF,respectively). At an elevated temperature (e.g., T=70°), thedecomposition of formate in acidic environment runs most efficientlywhen the ratio potassium formate to formic acid lies in the range from10:0.5 to 10:10, especially from 10:1 to 10:5, e.g. from 10:1 to 10:3.

Example 17 Formate Decomposition in a Highly Concentrated Solution inthe Presence Formic Acid

To illustrate that acidic environment generated by FA is capable ofadvancing formate decomposition also in highly concentrated PFsolutions, FA was added to 16M PF aquatic solution at ratio FA:PF 1:10(giving a solution of 16M PF, 1.6M FA). This solution went throughdehydrogenation over Pd/C (Sigma 205680) at 70° C. Initial TOF=178min⁻¹, TON (at deactivation)=646.

Examples 18-24 Formate Decomposition in Acidic Environment at 25° C.

The ability of formic acid to advance formate decomposition at 25° C.was studied. Various amounts of FA were added to PF 4M aquaticsolutions. These solutions went through dehydrogenation over Pd/C (Sigma205680) at 25° C. The conditions of the reactions and the performance ofthe catalyst in the presence of formic acid at 25° C. are tabulated inTable 3.

TABLE 3 Example PF:FA ratio TON at deactivation TOF (min⁻¹) 18 10:1 25917 19 10:2 353 32 20 10:3 615 33 21 10:4 752 29 22 10:5 676 31 23 10:6613 30 24  10:10 313 30

The results show that the catalytically-driven decomposition of formateprogresses satisfactorily even at room temperature, with the aid offormic acid. On graphically presenting the results in a bar diagram,where the abscissa and ordinate are MR_(PF:FA) and TON, respectively, acurve resembling inverted parabola is seen (FIG. 6), with an axis ofsymmetry in the range of 10:3<MR_(PF:FA)<10:5.

Examples 25-27 Formate Decomposition in Acidic Environment at 25° C.

The experimental procedures set forth in the previous set of exampleswere repeated, but this time with higher concentration of PA, andcorrespondingly, with higher concentration of FA. The solutions wentthrough dehydrogenation over Pd/C (Sigma 205680) at 25° C. Theconditions of the reactions and the performance of the catalyst in thepresence of formic acid at 25° C. are tabulated in Table 4.

TABLE 4 PF concentration PF:FA TON at Example (M) molar ratiodeactivation TOF (min⁻¹) 25 12.0 10:2 519 50 26 12.0 10:3 563 88 27 14.5  10:1.2 344 47

It can be seen that formic acid promotes the catalytically-drivendecomposition of highly concentrated formate solutions at roomtemperature. The results set out in Table 4 are shown graphically inFIG. 7, where the TON is plotted as function of reaction time for eachof the three experiments of Examples 25, 26 and 27, illustrating thatTONs (at deactivation) higher than 300 are achievable in the very highconcentration regimen of formate aqueous systems, with fairly reasonablePF:FA molar ratio.

Example 28 Formate Decomposition in Acidic Environment at 25° C. and inNeutral pH at 70° C.

PF and FA were added to water to form an aqueous solution with PF and PAconcentrations of 12.0M and 2.4M, respectively. The catalytically-drivenreaction started at room temperature in the presence of Pd/C (Sigma205680, the catalyst loading was 1:500 relative to the PF). After 90minutes, the acid was essentially consumed (as indicated by cessation ofhydrogen evolution) and the reaction mixture was heated to 70° C. andkept at that temperature for about additional 90 minutes to reach almostfull decomposition of the formate.

A curve showing the TON (left ordinate) and percentage of formateconversion (right ordinate) as function of reaction time is plotted inFIG. 8.

Examples 29-35 Formate Decomposition in Acidic Environment at DifferentTemperatures

The effect of temperature variation on formate decomposition in anacidic environment induced by formic acid was tested. Aquatic solutionsof potassium formate 4M an formic acid 0.8M went through dehydrogenationover Pd/C 5% (Sigma 205680) at a variety of temperatures, as describedin Table 5:

TABLE 5 Example Temperature (° C.) TON at deactivation TOF (min⁻¹) 29 0° 519 9 30 10° 645 23 31 20° 858 41 32 30° 799 46 33 40° 1110 85 3450° 632 63 35 60° 726 101

A bar diagram showing the results of TON at deactivation as function oftemperature is given in FIG. 9.

Examples 36-37 Hydrogen Release from Potassium Formate Solution OverPd/C 0.2% and Commercial Pd/C 5%

10 ml of 4M solution was prepared by addition of water to 3.3648 g (0.04mol) of potassium formate. The solution was added to 0.59 g of Pd/C 0.2%(40% wet, 0.0067 mmol) of Preparation 7 or 0.014 g of Pd/C 5% (Sigma205680, 0.0067 mmol) and went through dehydrogenation at 70° C.(formate:Pd molar ratio of 6000:1). The profile of the reaction isillustrated in the graph of FIG. 10, where the turn over number (TON) isplotted against time. The results indicate that hydrogen can be releasedfrom potassium formate solution over Pd/C 0.2% with TON over 7 timesbetter than when using Pd/C 5%.

Preparation 1 Supported Palladium Catalyst

CNT (commercial multiwall carbon nano-tubes) or activated carbon wasplaced in a flask with isopropyl alcohol (IPA). The mixture wassubjected to sonication for a total of 20 minutes (activation periods ofone seconds each, with an intermission of one second between eachactivation period). Palladium acetate solution in IPA was prepared andadded to the flask. The flask was heated to reflux (85° C.) for 3 hoursfollowed by evaporation of the IPA. The content of the flask was driedfor 1 hour under vacuum at 65° C.

Preparation 2 Supported Palladium Catalyst

Montmorillonit k-10 and di-n-decyldimethylammonium bromide were placedin a flask (weight ratio ammonium salt:mineral 5:3). Ethanol was addedas a solvent for the ammonium salt. The mixture was stirred at roomtemperature for 3 hours and then filtered and washed with ethanol. Theammonium salt absorbed-montmorillonit was placed in water with palladium(II) nitrate (weight ratio palladium:mineral 1:9). The mixture wasstirred for 18 hours, then filtered, washed with water and dried atvacuum at 60° C.

Preparation 3 Supported Palladium Catalyst

Palladium (II) acetate was dissolved in acetone. CNT or activated carbonwas added to the solution according to the desired percentage ofpalladium. The mixture was stirred while aquatic solution of hydrazinewas added dropwise for 30 minutes. The mixtures were left over night andfiltered by gravitation the next morning.

Preparation 4 Supported Palladium Catalyst

CNT, activated carbon or PANI (polyaniline) was placed in water withpalladium (II) nitrate. A reductive agent such as hydrazine solution orsodium borohydride aquatic solution was added dropwise for 30 minutes.The mixture was stirred for 18 hours, then filtered, washed with waterand dried at vacuum at 60° C.

Preparation 5 Supported Palladium Catalyst

CNT, activated carbon or PANI (polyaniline) was placed in water withpalladium (II) nitrate. Hydrogen gas was added to 9.5 atmospheres for 2hours at room temperature. Then the mixture was filtered, washed withwater and dried at vacuum at 60° C.

Preparation 6 Supported Palladium Catalyst

Palladium (II) nitrate dihydrate (0.096 mmol, Sigma 76070) was dissolvedin water (1 L). Activated carbon (Sigma C-3345) was heated to 200° C.for 1 hour. The treated activated carbon (25 g in order to get 0.4%Pd/C) was added into the palladium solution and stirring was activatedto 700 rpm for 1 hour. Then an aqueous solution of potassium formate(0.081 g in 200 ml of water) that was used as a reduction agent wasadded dropwise for 30 minutes at 25° C. (molar ratio between palladiumand reduction agent is 10:1, total concentration of formate in thevessel was 0.008M). Following that the mixture was left while stirringcontinued at room temperature for 24 hours. After 24 hours the mixturewas filtered, washed thoroughly with deionized water and left to dry atroom temperature.

Preparation 7 Supported Palladium Catalyst

Pd/C 0.2% was prepared using a procedure similar to that of Preparation6, i.e., via formate reduction of Pd²⁺ under mild conditions, but thistime 0.5 g of the treated activated carbon were added to the palladiumsalt solution to achieve the 0.2% loading.

The Pd/C powder collected was subjected to STEM-EDS analysis [Tecnai F20G2 (FEI company)]. FIG. 11 is a STEM image showing a region of the Pd/Csample which contains palladium (36.3%). The Pd particles in the regionobserved are invisible, indicating that the palladium present in thatregion consists of sub-nanometer Pd particles (below TEM resolution).There was no indication of the presence of potassium in thesub-nanometer Pd particles regions.

On the other hand, in the STEM image of FIG. 12, another region of thePd/C sample is shown, where Pd particles in the low nanometer range ofsize (from 1 nm to 5 nm) are identified. Compositionally, the palladiumcontent is 65.59% and the potassium content is 0.55% (w/w).

The invention claimed is:
 1. A Pd/C catalyst wherein Pd loading is inthe range from 0.15 to 0.5 wt %, and wherein at least a portion of thepalladium is present on the support in the form of sub-nanometerparticles (<1 nm).
 2. The Pd/C catalyst according to claim 1, whereinthe Pd loading is from 0.2 to 0.5 wt %.
 3. A process for preparing Pd/Ccatalyst according to claim 1, comprising dissolving in water apalladium (Pd²⁺) salt, adding to the solution heat-treated activatedcarbon, stirring the so-formed mixture, reducing the Pd²⁺ to Pd⁰ withthe aid of formate, collecting a powder consisting of Pd/C, washing anddrying same, wherein the Pd loading is in the range from 0.15 to 0.5 wt%, and at least a portion of the palladium is present on the support inthe form of sub-nanometer particles (<1 nm).
 4. The process according toclaim 3, wherein the activated carbon bears acidic groups.
 5. A processaccording to claim 4, wherein the formate is potassium formate and theconcentration of said formate in the reaction mixture is from 0.005 to0.12 M.
 6. The process according to claim 3, wherein the formate ispotassium formate and the potassium is incorporated into the Pd/Ccatalyst, and wherein regions of the Pd/C catalyst where sub-nanometerPd is present are potassium-free.
 7. The Pd/C catalyst according toclaim 1, wherein the carbon support of the Pd/C catalyst is activatedcarbon bearing acidic groups.
 8. The Pd/C catalyst according to claim 1,wherein potassium is incorporated into the Pd/C catalyst, and whereinregions of the Pd/C catalyst where sub-nanometer Pd is present arepotassium-free.
 9. A reaction medium for catalytic production ofhydrogen comprising: an aqueous potassium formate solution, an acid, anda catalyst according to claim 1, wherein concentration of the aqueouspotassium formate solution is not less than 4 M, and the molar ratio ofpotassium formate to the acid is from 10:1 to 10:10.
 10. A reactionmedium according to claim 9, wherein the molar ratio of potassiumformate to the acid is from 10:2 to 10:6.
 11. A reaction mediumaccording to claim 9, wherein the acid is formic acid.
 12. A reactionmedium according to claim 11, wherein the molar ratio of potassiumformate to the acid is from 10:1 to 10:3.
 13. A reaction mediumaccording to claim 9, wherein the ratio of potassium formate topalladium catalyst is 2000:1.
 14. A reaction medium for catalyticproduction of hydrogen comprising: an aqueous potassium formate solutionand a catalyst according to claim 1, wherein concentration of theaqueous potassium formate solution is not less than 4 M, and the molarratio of potassium formate to palladium catalyst is from 2000:1 to6000:1.